The spread spectrum communication system is such a one in which a transmitter modulates a transmission signal and spectrum-spreads the modulated transmission signal using a spread code (pseudorandom noise, herein abbreviated as “PN”), to transmit the spectrum-spread signal, and in which a receiver upon receiving and demodulating the spectrum-spread signal, transmitted from the transmitter, inverse-spreads (despreads) the received spectrum-spread signal, using the same PN sequence as that used for spreading in the transmitter. It is only when a PN sequence contained in the received signal coincides with that generated on the receiver side that correct demodulation is achieved.
Recently, the CDMA (Code Division Multiple Access) communication system, in which a PN sequence of the spread-spectrum system is allocated from one communication event to another, is felt to be promising as a standard for a mobile terminal equipment of a mobile communication system for the wireless (radio) communication system. In this CDMA communication system, the user information pieces, spectrum-spread by a PN specific to a user, are transmitted as they are superposed in the same frequency band, and, on the other hand, a receiver extracts the information using the PN specific to the user desiring to receive. This CDMA communication system has a number of merits, such as high spectrum utilization efficiency, invulnerability to multiple paths and high secrecy.
In the communication system of this CDMA system, the generating timing of the PN sequence contained in received signals and the generating timing of the PN sequence Provided on the receiving side are estimated with a precision within one chip, and the operation of a PN sequence generator is started at this timing, by way of synchronized capture (synchronization acquisition), in order to achieve timing synchronization with respect to the PN contained in the signal. Moreover, in the direct spread (DS) system, since a slightest deviation from the synchronization Position leads to failure in tracking (or tracing) of the received signal, synchronization tracking for monitoring to prohibit time shift of the PN sequence is used on the receiver side with respect to the received signal once acquired with success. To this end, a Preset fixed pattern is inserted as a synchronization signal by a transmitter into a transmission signal for transmission, whilst a receiver calculates a correlation value between the received signal and the fixed pattern by way of synchronization detection to detect received signals and/or to effectuate frame synchronization, for synchronization control.
In a correlator for synchronized capture by a receiver of the spread-spectrum communication, the received signal is multiplied with a PN sequence chip-by-chip and the results of multiplication are summed together to determine a correlation value. Since a high correlation value is obtained when a PN sequence of the received signal coincides with a PN sequence provided on the receiver side, correlation is sought as a PN sequence of the received signal is being shifted in phase from the PN se sequence of the receiver side. The position of a maximum correlation value or the position which has exceeded a Preset threshold correlation value is used as a position of synchronization to effectuate demodulation at a corresponding phase. Among the methods based on the Phase shifting, such a method is used in which the PN sequence generated on the receiver side is shifted as in a matched filter method or sliding correlation method.
The correlator by the matched filter includes shift registers, plural multipliers for multiplying outputs of respective stages of the shift registers with a PN and an adder for summing outputs of the plural multipliers. The spread-spectrum signal, which is a signal received e.g., over an antenna and subsequently converted into a base-band signal is sequentially stared chip-by-chip in the shift registers. The spread-spectrum signal stored in each stage of the shift registers is multiplied chip-by-chip with the PN in each multiplier and the multiplied results are transmitted to an adder to obtain a sum which is sent out as an output signal. If the PN sequence coincides in timing with the PN sequence of the received spread-spectrum signal, an output of the adder assumes a maximum value signal (matched pulse). So, the synchronization is detected from the matched pulse and, based on the results of synchronization detection, demodulation is performed by a demodulator. As for the synchronization detection circuit employing a carrelator by a matched filter, reference is had to the publication of the Japanese patent No. 2850959.